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The hyperfactorial is defined as $$H(n) = \prod^{n}_{i = 1} i^i = 1^1 \cdot 2^2 \cdot 3^3 \cdot 4^4 \cdot \ldots \cdot n^n$$.[1]

The first few values of $$H(n)$$ for $$n = 1, 2, 3, 4, \ldots$$ are 1, 4, 108, 27,648, 86,400,000, 4,031,078,400,000, 3,319,766,398,771,200,000, 55,696,437,941,726,556,979,200,000, 21,577,941,222,941,856,209,168,026,828,800,000, ... (OEIS A002109). The sum of the reciprocals of these numbers is 2.2592954398150629..., which can be approximated as $$\sqrt[12]{17,688}$$, or more precisely as $$\sqrt[7]{\sqrt[7]{3^{4}\cdot67\cdot3,929\cdot10,371,376,751}}$$, a curious 18-decimal-place approximation where we have a double 7th root (7 is prime) of the product of seven prime factors.

## Sources Edit

1. Hyperfactorial from Wolfram MathWorld
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